I really don't hope jumping from rooftop to rooftop is going to be Crackdown-style, or something similar. That wouldn't be GTA. Flying with motorbikes without falling, bunny-hopping 3 meters or getting shot a thousand times without dying doesn't annoy me But flying while being on foot does.

Think more like Assassins Creed.
If this info is indeed correct, I'm very very happy. Hanging on rooftops etc is great fun, and using the trunk is even moar fun.